1. Field of the Invention
Rapid production of prototypes is a task often encountered in very recent times. Particularly suitable processes are those whose operation is based on pulverulent materials and which produce the desired structures layer-by-layer via selective melting and hardening. Support structures for overhangs and undercuts can be avoided in these processes, because the powder bed surrounding the molten regions provides adequate support. Nor is there any need for subsequent operations to remove supports. These processes are also suitable for short-run production.
The invention relates to the use, in shaping processes, of a polyester powder which was prepared from a di- or polyhydric alcohol and from a dicarboxylic acid, while avoiding any aromatic monomer unit, and also to moldings produced via a layer-by-layer process by which regions of a powder layer are selectively melted, using this powder. After cooling and hardening of the regions previously melted layer-by-layer, the molding can be removed from the powder bed.
Selectivity of these layer-by-layer processes can by way of example be achieved by way of application of a susceptor, of an absorber, or of an inhibitor, or via a mask, or by way of focused introduction of energy, for example via a laser beam, or by way of glass fibers. Energy input is achieved by way of electromagnetic radiation.
There follows a description of some processes which, with the inventive use of a polyester powder, can produce inventive moldings, but there is no intention that the invention be restricted thereto.
2. Discussion of the Background
One process which has particularly good suitability for the purposes of rapid prototyping is selective laser sintering. This process irradiates plastics powders selectively and briefly with a laser beam in a chamber, the result being that the powder particles impacted by the laser beam melt. The molten particles coalesce and rapidly solidify again to give a solid mass. This process can produce three-dimensional products simply and rapidly via repeated irradiation of a succession of freshly applied layers.
The process of laser sintering, i.e., rapid prototyping, for production of moldings from pulverulent polymers is disclosed in U.S. Pat. No. 6,136,948 from DTM Corporation. A wide variety of polymers and copolymers is disclosed, examples being polyacetate, polypropylene, polyethylene, ionomers, and polyamide.
Other processes with good suitability are the selective inhibition of bonding (SIB) process as disclosed in U.S. Pat. No. 6,589,471 or a process disclosed in U.S. Pat. No. 6,531,086. Both processes operate with full-surface infrared heating to melt the powder. Selectivity of melting is achieved in the first process via application of an inhibitor, and in the second process via a mask. US 2004/232583 discloses another process, in which the energy needed for melting is introduced via a microwave generator, and selectivity is achieved via application of a susceptor.
Other suitable processes are those which operate with an absorber, which is either present in the powder or is applied via ink jet methods, as described in DE 10 2004 012 682.8, DE 10 2004 012 683.6, and DE 10 2004 020 452.7.
The rapid prototyping or rapid manufacturing processes mentioned (RP or RM processes) can use pulverulent substrates, in particular polymers, preferably selected from polyester, polyvinyl chloride, polyacetal, polypropylene, polyethylene, polystyrene, polycarbonate, poly(N-methylmethacrylimides) (PMMI), polymethylmethacrylate (PMMA), ionomer, polyamide, or a mixture thereof.
U.S. Pat. No. 5,342,919 discloses a polymer powder suitable for laser sintering which exhibits no overlap of the melting and recrystallization peak when melting behavior is determined via differential scanning calorimetry at a scanning rate of from 10 to 20° C./min, and which has a degree of crystallinity of from 10 to 90%, likewise determined via DSC, and has a number-average molecular weight Mn of from 30 000 to 500 000, its Mw/Mn quotient being in the range from 1 to 5.
U.S. Pat. No. 6,245,281 discloses the use of a nylon-12 powder with increased melting point and increased enthalpy of fusion, obtained via reprecipitation of a polyamide previously prepared via ring-opening and subsequent polycondensation of laurolactam. This is a nylon-12.
DE 10 2004 010 160 A1 describes the use of polymer powder with copolymer in shaping processes. These are thermoplastic random copolymers composed of a very wide variety of monomer units, the emphasis here being placed on laurolactam-based systems. Monomers are mentioned by way of example for copolyesters, but no details are given of specific constitutions. The melt flow rate (MFR) value of the copolymers is from 1 to 10 g/10 min.
One processing disadvantage is that in order to avoid what is known as curl the temperature in the construction space or construction chamber has to be kept with maximum uniformity at a level just below the melting point of the polymeric material. In the case of amorphous polymers, this means a temperature just below the glass transition temperature, and in the case of semicrystalline polymers this means a temperature just below the crystallite melting point. Curl means distortion of the region after melting, the result being at least some protrusion out of the construction plane. There is an associated risk that when the next powder layer is applied, for example via a doctor or a roller, the protruding regions may be shifted or even entirely broken away. The consequence of this for the process is that the overall construction space temperature has to be kept at a relatively high level, and that the volume change brought about via cooling and via crystallization of the moldings produced by these processes is considerable. Another important factor is that the period required for cooling is significant, especially for “rapid” processes.
Another disadvantage of the semicrystalline thermoplastics in many instances is their crystallinity, and the volume change caused thereby during cooling from the melt. Although it is possible to use very complicated and precise temperature control to achieve a substantial equalization of the volume change due to the crystallinity of an individual layer, the volume change due to crystallization in three-dimensional moldings of any desired structure is not uniform throughout the molding. By way of example, the formation of crystalline structures is dependent on the cooling rate of the molding, and at locations of different thickness or at angled locations this rate differs from that at other locations within the molding.
A disadvantage of amorphous thermoplastics is high viscosity, permitting coalescence only markedly above the melting point or the glass transition temperature. Moldings produced by the above processes using amorphous thermoplastics are therefore very often relatively porous; the process merely forms sinter necks, and the individual powder particles remain discernible within the molding. However, if the amount of energy introduced is increased in order to reduce viscosity there is the additional problem of dimensional accuracy; by way of example, the contours of the molding lose sharpness as a result of heat conducted from the melting regions into the surrounding regions.
One disadvantage of the copolymers previously disclosed in the literature for use in moldless shaping processes is that although melting points can be lowered by means of altering the composition with a resultant favorable effect on processing and on shrinkage. In particular, this is accomplished specifically by usage of at least one aromatic monomer unit, which results in a reduction in the crystallinity, with the result that the crystallite melting point then is not a measure of the transition from a solid to a liquid, but instead, it is substantially a measure of the glass transition, the transition being gradual and dependent on the constitutions of the copolymers. The aromatic monomer unit can be terephthalic acid or isophthalic acid, for example. However, a contrary effect is that these aromatic components markedly increase the viscosity of the melt, making coalescence of the powder particles more difficult. Therefore, a compromise always has to be found between competing targeted properties. The melting points of the homopolymers mainly used at present are above 160° C., examples being nylon-12 (melting point: 186° C.) and nylon-11 (melting point: 193° C.). A disadvantage here is that more curl arises and can even prevent processing, and that the requirements placed upon the machine are very much more stringent because uniformity of temperature in the construction area has to be maximized, and specifically at a level just below the melting point of the polymer.